I'm going to suggest salting the QT tank as well.
I've had tetras and catfish fine with salt although at a lighter dose for longer. I've thought it does help, even if it just seems to clean the fish up so the meds have a better chance.
On more reading a 1% dose rate is to aim for. If the fish rolls over, loses equilibrium or gets very slow gill movement, then a water change to reduce salt level.
I've copied in a link below for dosing. Mainly as I tend to think in gallons and tablespoons. I would dose 1 teaspoon salt per 1 gal
to get them used to it, then 3 after 12hrs (commonly suggested in US as 1 tablespoon per gal
max), then to the 7.5 per gallon as in link.
I'd let them go longer then 12hrs in it if doing ok, more like 48. It's going to be a bit of suck and see if you did want to try.
Only other med to try that I know is in lfs
is the tetracycline. It's not suited to higher ph and wrong bacteria class but out of options. I'd actually favour the salt first.